It is known to record or digitise individual signals or transients arising from ion arrivals at an ion detector or electron multiplier using an Analogue to Digital recorder or an Analogue to Digital Converter (“ADC”). Orthogonal acceleration Time of Flight mass spectrometers may digitise ion arrival signals or transients relating to many thousands of individual time of flight separations. The digitised signals or transients are summed to produce a final summed or composite time of flight mass spectrum. Each individual time of flight spectrum, signal or transient may be processed in real time before summing. In the simplest case this processing may be the application of an amplitude threshold to isolate signals arising from ion arrivals from background noise or baseline noise. The signal at an individual digitised sample (i.e. an individual Analogue to Digital Converter time bins) or within a time of flight spectrum which is above the threshold is recorded and all other samples or intensity values in Analogue to Digital Converter time bins are set to zero or to a baseline value. Such a method is disclosed, for example, in US 2011/0049353 (Micromass). Multiple time of flight spectra processed in this way may then be summed or averaged to generate a final summed spectrum with reduced noise.
It is also known to process individual signals or transients which have been digitised to reduce the ion arrival signals or transients into time and intensity pairs. Such a method is disclosed, for example, in U.S. Pat. No. 8,063,358 (Micromass). Individual signals or transients which are reduced to time and intensity pairs may then be summed with other time and intensity pairs relating to other time of flight spectra, signals or transients in order to produce a final summed, composite or average spectrum. This method substantially removes the profile or line width of the digitised signal from the final summed spectra thereby increasing the effective time of flight resolution. It also simplifies implementation of dual Analogue to Digital Converter approaches to extending dynamic range, such as U.S. Pat. No. 8,354,634 (Micromass), and allows simple up-sampling of output spectral data rates.
Other methods of reducing the contribution of the single ion pulse width are described in U.S. Pat. No. 6,870,156 (Rather).
In methods which involve reducing individual transients to time and intensity pairs, each ion arrival has an associated analogue peak width. If two or more ions arrive simultaneously then these analogue peak widths may partially overlap making it impossible for a simple Finite Impulse Response filter, peak maxima or related peak detection method to isolate the arrival time and intensity of the individual ions. In such a case a response related to the average ion arrival time and summed area may be recorded rather than two individual ion arrival times and intensities.
This coalescing of two or more ion arrivals within a transient into a single time intensity pair can cause artefacts in the final summed data. Furthermore, the analogue peak width from ions of different mass to charge ratio species may overlap significantly within a single transient. This will result in an inaccurate representation of the signal intensity and an inaccurate measurement of the ion arrival time for each mass to charge ratio species.
A method of de-convolving such overlapping signals is described in U.S. Pat. No. 8,735,808 (Micromass). However, this method can be computationally intensive.
GB-2457112 (Micromass) discloses methods and apparatus for detecting ions.
GB-2506714 (Micromass) discloses calibrating dual ADC acquisition systems.
GB-2439795 (Micromass) discloses obtaining mass spectra from a Time of Flight mass spectrometer.
WO 98/21742 (Rockwood) discloses a multi-anode time to digital converter.
EP-2447980 (Makarov) discloses a method of generating a mass spectrum having improved resolving power.
US 2005/0114042 (Pappin) discloses a method and apparatus for deconvoluting a convoluted spectrum.
US 2004/0083063 (McClure) discloses a method and apparatus for automated detection of peaks in spectroscopic data.
It is desired to provide an improved method of mass spectrometry and an improved mass spectrometer.